Assuming you are not using autoscaling, I believe this should work:
fly scale count 3 --max-per-region 1
Give it a minute and then check to see where the vms are. I guess a vm still may not be created in a chosen region (like if the region is having issues, capacity etc). But if all is well, it should be.
If you are using autoscaling, it’s a bit more complicated because of how the innards work. This thread details some of the complexity with autoscaling and distribution: